Digital radio systems offer users versatile services that require that the radio system be able to transfer speech and data at high rates. The nature of most services is still such that the need for data transmission is greater in the downlink direction, i.e. from the radio system to a terminal, such as when Internet-based browsers are used. Furthermore, the nature of data services is such that the need for data transmission is transient, and it is therefore not advantageous to continuously reserve a high capacity for a user from the point of view of efficient utilization of the resources of the radio system.
The duplex method TDD (Time Division Duplex) used in digital radio systems is a partial answer to the needs of the type described above and caused to a radio system by data traffic. In TDD systems, the uplink and downlink directions are separated from each other in time, and operate in the same frequency range. In some TDD-based systems, the borders between the transmission directions are not exactly defined, but more radio resources, such as timeslots, can be allocated to for example the downlink direction, if required. Furthermore, digital radio systems comprise various channels reserved for different purposes. Some channels are called dedicated channels, whereby data transmission resources, such as a given combination of a radio frequency, a timeslot and a spreading code, are reserved for data transmission between the radio network and a terminal. Some channels, in turn, are commons channels, whereby no data transmission resources are reserved between the radio network and a terminal, but all terminals can listen to all channels. In this case the radio system may comprise, for example, a common traffic channel shared by several users, on which a terminal can receive information at the same time as the terminal communicates in the radio network on a dedicated channel. A shared channel is particularly well suitable for use for data traffic, since it allows the capacity offered by a dedicated channel having a low data transmission capacity to be increased.
In some digital radio systems, information to be transmitted on radio channels is arranged into bursts, which are information packets in a specified format. An alternative to traffic in bursts is continuous transmission on a radio channel in a radio system. Depending on the channel, the information to be transmitted in bursts may contain either user data or control information associated with the use of the radio system; often both. The structure of a normal burst, used for example in data transmission, is such that in the middle the burst comprises a training sequence composed of a number of predetermined symbols known to the terminal. On both sides of the training sequence are data periods, and the burst further comprises guard periods for separating the burst from other bursts. The receiver compares the received training sequence with a known training sequence, and, on the basis thereof, is able to better demodulate the received signal. It is also known to insert into the burst an indicator of the length of a few data elements, such as a TFCI (Transport Format Combination Indicator), to supply the terminal with information on the use of the radio network, such as user bit rate. The TFCI indicator further allows for example the receiver of a burst to be indicated on a shared channel. Another way to transmit control information of the above type to a terminal is to use a control channel reserved for that purpose.
However, known methods have drawbacks. The use of TFCI indicators of the length of some bits is not necessarily adequate to reliably transmit to a user the required control information, because of interference at the radio interface. The use of TFCI bits in bursts further decreases the data transmission capacity of the system, since there is less space for actual user data in the bursts. The use of higher-level signalling for transmitting control information also takes up system capacity, since the control signals to be transmitted through the system require measures of several sub areas of the radio system.